This research project will determine the rate of serious and persistent Graves' eye disease (GED) developing within one year after treatment of hyperthyroidism with 131-I in a clinical practice within the United States. Comparison will be made with the relatively high rate of GED developing or worsening in patients treated with radioiodine in Italy. All patients with Graves's hyperthyroidism who are 18 years of age and older and are referred for radioiodine treatment will be recruited over a 2 year period. They will be of both sexes and of all ethnic origins. The patients will be asked to undergo selective ophthalmologic examinations (limited to known features of GED) on 4 occasions over the subsequent year, beginning before the radioiodine treatment. The patients will give blood at the outset for measurement of serum triiodothyronine, and at the outset and the end of the protocol for thyroid stimulating immunoglobulin. The values from these tests may correlate with, and therefore may predict, the development of GED. Patients also will fill out a brief quality-of-life (related to the eyes) questionnaire at the outset and at the end of the protocol; the results of the questionnaires may give data that bear on the seriousness of any GED that would be in addition to the evaluation by the ophthalmologists. Treatment of hyperthyroidism will be outside the protocol and according to standard medical practice.If the rate of development of GED is less than 2%, the results of this project will be reassuring to physicians and patients alike. If the rate of development of GED in patients treated in the United States significantly differs from the rate reported from Italy and elsewhere, a search for the cause of the difference may provide insights into the pathogenesis of GED.